1. Field of the Invention
This invention relates to a communications protocol useful in digital telemetry systems such as in well-logging applications or surface seismic measurement.
2. Background
Well-logging is the measurement of characteristics of different earth formations traversed by a borehole, usually an oil or gas well using one or more measuring instruments or tools. The tools are typically stacked in a tool string, the tool string being attached to a logging cable which supports the tool string, provides power to the tool or tools and provides a communication medium for the transmission of data from the tool or tools to data acquisition and processing equipment on the surface.
Data transmitted in a well-logging digital telemetry system are typically first transmitted over a bus within the tool string to a downhole modem. The downhole modem then uses that data to modulate a carrier signal suitable for transmission over the logging cable to the surface.
Digital communications systems require a detailed set of rules and operating procedures which enable data to be communicated effectively, namely a protocol. Communications protocols include procedures for framing, error control, sequence control, line control, and start-up control. Framing refers to the manner in which certain groups of bits are identified as either control bits or actual message bits. Error control refers to the detection of errors, acceptance and acknowledgment of correct messages, and requests for retransmission of faulty messages. Sequence control refers to the identification of messages that are retransmitted by the error control system in order to avoid loss or duplication of messages. Line control refers to the manner in which transmit/receive time windows for each transmitter or receiver are defined. That is, each transmitter or receiver in the system must know when it is to transmit or receive data. Start-up control refers to the procedure to be followed in order to start the system operating from an idle state.
Another aspect of communications protocols particularly important in well-logging applications is the manner in which messages are tagged with the depth at which the tool was located when the message was written by the tool. That may be done, in accordance with the present invention, by tagging each message with a clock value representing the time at which the message was written. Tool depth can be separately correlated with time, and it is therefore possible to calculate a tool depth for each message. Tagging each message with a clock value is referred to as time stamping.
One standard for evaluating a communications system is how much data can be transmitted over the system in a given time. The effective data rate of the system after allowance for protocol and half-duplex operation (as distinguished from the transmission rate of any individual transmitter) is the "bandwidth" of the system. An object of the present invention is to provide communications protocols that maximize the system bandwidth while at the same time minimizing the error rate by detecting and correcting those messages containing errors.
A system parameter related to bandwidth is latency. Latency is the round-trip time for uplink tool data to reach the surface and a downlink message to be received by the tool. In half- duplex communications systems, one factor influencing latency is the time required for the communications link to switch directions. Uplink bandwidth, downlink bandwidth, and latency are related. Downlink bandwidth can be increased at the expense of uplink bandwidth by allowing the system to transmit downward more of the time and vice versa. If either the uplink or the downlink transmitter does not actually use all the bandwidth allocated to it, latency will be lower than it otherwise could be.
Another parameter of digital communications systems is transmitter buffering capacity. Some communications systems use retransmission to reduce the error rate of the system. That is, when faulty data are received, the transmitter simply retransmits the data. The data must therefore be stored in buffers for some period of time by the transmitter until an acknowledgment that the data has been received without error is received. Buffering capacity, however, is expensive, and it is advantageous to minimize the buffering capacity needed to achieve a given error rate.
The ease of implementation of a particular protocol is also important. If a protocol is difficult or expensive to implement, it has little value. The best communication protocols provide the desired features with minimal overhead and cost-effective implementation.